Various processes have been known and utilized in the prior art for the separation of air into its nitrogen and oxygen dominant constituents. Additionally, the use of a single pressure distillation column is known to have been used in the prior art for such separations.
U.S. Pat. No. 2,627,731 discloses a process for the rectification of air into oxygen and nitrogen, wherein a two sectioned or single distillation column are used alternatively. Air is cooled by heat exchange and introduced directly into the distillation column. A nitrogen product is removed from the overhead of the column and a portion is compressed in two stages. The first stage nitrogen compressed stream is recycled in order to reboil and condense a portion of the midpoint of the column by indirect heat exchange before being introduced into the overhead of the column as reflux. A second stage compressed nitrogen stream is recycled and partially expanded to provide refrigeration. This expanded stream is recycled to the nitrogen product line. The remaining stream of the second stage compressed nitrogen stream reboils the bottom of the column before being combined with the first stage compressed nitrogen stream and introduced into the overhead of the column as reflux.
U.S. Pat. No. 2,982,108 discloses an oxygen producing air separation system wherein a portion of the nitrogen generated from the distillation column is compressed and reboils the base of a high pressure section of the column before being introduced as reflux to the low pressure section of the column. The feed air stream is supplied in separate substreams into the high pressure section of the column and in an expanded form into the low pressure section of the column.
U.S. Pat. No. 3,210,951 discloses a fractionation cycle employing first and second fractionating zones operating under different pressures and including two reboiler/condensers. Both of the reboiler/condensers are interconnected with the stages of fractionation in such a manner as to effect the required reboil and reflux production with minimum pressure differential between the stages of rectification and also decreased the irreversibility of the overall fractionation process thereby obtaining the desired separation with the high pressure stage operating under substantially reduced pressure.
U.S. Pat. No. 3,214,926 discloses a method for producing liquid oxygen or liquid nitrogen. However, in the patent it is necessary to have two distillation columns, one at high pressure and another at low pressure in order to extract liquid oxygen.
U.S. Pat. No. 3,217,502 discloses a system which utilizes a single pressure distillation column. The product of this air separation system is gaseous and liquid nitrogen. Impure oxygen which is separated out in this system is vented to waste. In this patent, it is the oxygen waste stream which is expanded in order to provide refrigeration for the air separation system.
U.S. Pat. No. 3,277,655 discloses an improvement to the fractionation process taught in U.S. Pat. No. 3,210,951. In this process, the heat exchange occurring in one of the two reboiler/condensers between the bottoms liquid from the lower pressure column and the gaseous material from the high pressure column results in complete vaporization of the liquid from the low pressure column thereby satisfying the reboiler requirements of the low pressure column. Additionally, when the liquefied gaseous material from the high pressure column is introduced into the lower pressure column it improves the reflux ratio in the upper portion of the low pressure column which increases the separation efficiency and makes it possible to lower the pressure of the gaseous mixture entering the cycle.
U.S. Pat. No. 3,327,489 discloses another improvement to U.S. Pat. NO. 3,210,951 to lowr the pressure in the high pressure fractionator. In the process, the pressure reduction is obtained along with the associated power reduction by establishing a heat exchange between gaseous material, which may comprise the feed mixture, and a liquid component collecting in the bottom of the low pressure fractionator, with the liquid component being under different pressure.
U.S. Pat. No. 3,492,828 discloses a process for the production of oxygen and nitrogen from air wherein a nitrogen recycle stream is compressed and condensed in a reboiler in the base of a distillation column before being reintroduced into the column as reflux. A portion of the nitrogen recycle stream may be expanded in which the power provided by the expansion drives the compressor for the main nitrogen recycle stream.
U.S. Pat. No. 3,731,495 discloses an air separation system using an air feed compressor which is powered by combustion gases directed through a turbine. The turbine exhaust heats boiler steam to supplement the compressor drive. Electric generation is also considered. In addition, this reference utilizes two separate columns at separate pressures for the recovery of the individual gaseous components of air which are separated.
U.S. Pat. No. 3,735,599 discloses a control system for an air separation apparatus which comprises a reversing heat exchanger, an air liquefier, a single column rectifier provided with a condenser-evaporator and a cold generation device. In the apparatus, air is cooled in the reversing heat exchanger and liquefied in the air liquefier, the liquefied air is rectified in the single column to separate into liquid air abundantly containing oxygen and highly pure nitrogen.
U.S. Pat. No. 3,736,762 discloses a process for producing nitrogen in gaseous and liquefied form from air. A single distillation column is refluxed with nitrogen product condensed in an overhead condenser operated by the reboil of impure oxygen conveyed from the bottom of said column. At least a portion of the impure oxygen from the overhead condenser is expanded to produce refrigeration for the process.
U.S. Pat. No. 3,754,406 discloses a process for the production of low purity oxygen, in which a low pressure stream of incoming air is cooled against outgoing gas streams and fed into a high pressure distillation column. A high pressure stream of incoming air is cooled against outgoing gas stream, partially condensed against boiling oxygen product in a product vaporizer, and separated into gas and liquid streams. The liquid stream being subcooled and expanded into a low pressure fractionating column. The gas stream is reheated and expanded to provide process refrigeration and is introduced into the low pressure fractionating column. Crude liquid oxygen from the bottom of the high pressure column is cooled and introduced into the low pressure column after being used to liquefy some of the nitrogen from the high pressure column in an external reboiler condenser. Liquid oxygen product from the low pressure column is pumped to a higher pressure before being passed to the subcooler and the product vaporizer. The remainder of the high pressure nitrogen is liquefied in a second external reboiler/condenser and is used as reflux for the two columns. A waste nitrogen stream is removed from the low pressure column.
U.S. Pat. No. 4,222,756 discloses a process in which a two pressure distillation column is used in which both pressurized column sections are refluxed with a nitrogen-enriched stream. The low pressure column is fed by a oxygen-enriched stream from the high pressure column which is expanded to reduce its pressure and temperature.
U.S. Pat. No. 4,224,045 discloses a process where oxygen is produced by distillation of liquefied air in a two column unit. A gas turbine, powered in part by a nitrogen product stream, supplies the energy to compress the feed air.
U.S. Pat. No. 4,382,366 discloses an air separation system for the recovery of pressurized, substantially pure oxygen gas. The system uses a single pressure distillation column and burns a nitrogen-oxygen waste stream to provide power for the air compressor, the oxygen product compressor and electric generation. The distillation column has a split feed to develop reflux and reboil and to provide initial separation of the liquid and vapor components of the column.
U.S. Pat. No. 4,400,188 discloses a nitrogen production process wherein a single nitrogen recycle stream refluxes a distillation column which is fed by a single air feed. Waste oxygen from the column is expanded to provide a portion of the necessary refrigeration.
U.S. Pat. No. 4,464,188 discloses a process and apparatus for the separation of air by cryogenic distillation in a rectification column using two nitrogen recycle streams and a sidestream of the feed air stream to reboil the column. One of the nitrogen recycle streams is expanded to provide refrigeration and to provide power to compress the feed air sidestream.
U.S. Pat. No. 4,464,191 discloses an arrangement of distillation columns for subambient distillation of mixtures of two non-condensable gases. The two column arrangement which exchange liquid achieves a given level of separation over a smaller temperature range than a single column producing the same separation. The arrangement of the patent is particularly useful for air separation to produce medium purity (90 to 99%) oxygen and/or nitrogen.
U.S. Pat. No. 4,560,397 discloses a process for the production of ultra high purity oxygen and elevated pressure nitrogen by the cryogenic rectification of air wherein the product oxygen is recovered from a secondary column at a point above the liquid sump while impurities are removed from the column at a distance from the product withdrawal point.
U.S. Pat. No. 4,617,036 discloses a process for the cryogenic distillation of air to recover nitrogen in large quantities and at relatively high pressure, wherein a portion of the nitrogen reflux for the distillation is achieved by heat exchanging nitrogen gas in a side reboiler against waste oxygen at reduced pressure.
U.S. Pat. No. 4,617,037 discloses a nitrogen production method wherein air is compressed, is removed of water and carbon dioxide contained therein, and is simultaneously cooled to a temperature close to the liquefying point. The resultant cleaned and cooled air is fed into a rectifying column for rectification so that high purity nitrogen is removed from the rectifying column overhead and the oxygen-enriched liquid air is withdrawn from the rectifying column bottom and is expanded and fed into a condensation step wherein it becomes a source of reflux for the rectifying column and a source of refrigeration. In the method of the patent, a closed circulating cycle provides additional refrigeration.
U.S. Pat. No. 4,655,809 discloses an air separation system for the recovery of pressurized, substantially pure oxygen gas. The system uses a single pressure distillation column and utilizes the nitrogen product stream to provide power for feed air compression, segregated heat pump fluid compression, and electric generation. The system utilizes a segregated heat pump cycle which provides heat exchange for both column reboil and reflux.
U.S. Pat. No. 4,662,916 and 4,662,917 disclose variations on a process for the separation of air by cryogenic distillation in a single column to produce a nitrogen product and an oxygen-enriched product. In the process, at least a portion of the nitrogen product is compressed and recycled to provide reboil at the bottom of the distillation column and to provide some additional reflux to the upper portion of the column. In addition, part of the compressed air stream is expanded to provide work, which is used to drive an auxiliary compressor for recycle nitrogen stream compression.
U.S. Pat. No. 4,662,918 discloses a process for the separation of air by cryogenic distillation in a single column to produce a nitrogen product and an oxygen-enriched product. In the process, at least a portion of the nitrogen product is compressed and recycled to provide reboil at the bottom of the distillation column and to provide some additional reflux to the upper portion of the column. In addition, part of the compressed nitrogen recycle stream is expanded to provide work.
U.S. Pat. No. 4,702,757 discloses a process utilizing high and low pressure distillation columns for the production of an oxygen-enriched air product. Feed air is fed to the main heat exchangers at two pressures. The high pressure feed air from the main exchanger used to supply refrigeration, by expanding a portion of the high pressure air prior to introducing that portion into an intermediate location in the low pressure column, and to vaporize the oxygen-enriched air product prior to using the stram as reflux for the high pressure column. The low pressure feed air from the main heat exchangers is partially condensed to supply reboiler duty to a low pressure column and is then fed to a high pressure column. The high pressure column condenser is used to reboil an intermediate liquid in the low pressure column.
U.S. Pat. No. 4,704,147 discloses a process for the production of an oxygen-enriched air product, feed air is fed to the main heat exchangers at two pressures. The high pressure feed air from the main exchanger is partially condensed to vaporize the oxygen-enriched air product. This partially condensed feed air is separated with the vapor phase being warmed and expanded to supply refrigeration and subsequently being fed to the low pressure fractionation section, and the liquid phase being used to reflux both the high pressure and low pressure fractionation sections of a double distillation column. The low pressure feed air from the main heat exchangers is fed to the high pressure fractionation section. The high pressure fractionation section condenser is used to provie reboiler duty to the low pressure fractionation section.
U.S. Pat. No. 4,704,148 discloses a process utilizing high and low pressure distillation columns, for the separation of air to produce low purity oxygen and waste nitrogen streams. Feed air from the cold end of the main heat exchangers is used to reboil a low pressure distillation column and to vaporize the low purity oxygen product. This heat duty for column reboil and product vaporization is supplied by splitting the air feed into at least three substreams. One of the substreams is totally condensed and used to provide reflux to both the low pressure and high pressure distillation column, preferably the substream which is fed to the oxygen vaporizer, while a second substream is partially condensed with the vapor portion of the partially condensed substream being fed to the bottom of the high pressure distillation column and the liquid portion providing reflux to the low pressure column. The third substream is expanded to recover refrigeration and then introduced to the low pressure column as column feed. Additionally, the high pressure column condenser is used as an intermediate reboiler in the low pressure column.